Reduced volume gas spring or surge tank for cryocooler

ABSTRACT

A fibrous packing material ( 210 ) is distributed throughout the interior space of a surge tank ( 118 ). The packing material ( 210 ) makes the compression and expansion of gas within the surge tank ( 118 ) relatively more isothermal as compared to an empty tank. Thus, a smaller volume tank can be used without sacrificing performance. As a result, the size and weight requirements of the surge tank ( 118  ) have been reduced.

FIELD OF THE INVENTION

This invention relates in general to surge tanks for pulsetube cryocoolers and to fibrous packing for such surge tanks.

BACKGROUND OF THE INVENTION

Pulsetube cryocoolers are well-known devices that generate refrigeration to very low temperatures using an oscillating gas. In such a device, pressurized gas (usually helium) in a regenerator/pulsetube assembly is rapidly pulsed such that compression work is done in a warm region of the assembly to remove heat, and expansion work is done in a cold region to absorb a thermal load. Typically, pulsetube cryocoolers employ a surge tank (which is sometimes called a reservoir tank), which acts as a gas spring (which is sometimes called a compliance space). In some multiple stage pulsetube cryocoolers, multiple surge tanks are employed.

FIG. 4 shows a conventional cryocooler 400. The cryocooler 400 includes a pressure wave generator 402, or compressor, an ambient heat exchanger 410, a regenerator 404, a cold heat exchanger 412, and a pulsetube 414. The pulse generator 402 is coupled to one end of the regenerator 404. Coupled to another end of the regenerator 404 is a pulsetube 414. An inertance tube 416 is coupled to the pulsetube 414 as shown. A surge tank 418, or reservoir tank, is coupled to the inertance tube 416 such that the inertance tube 416 joins the surge tank 418 to the pulsetube 414. As shown, the pressure wave generator 402, the regenerator 404, the pulsetube 414, the inertance tube 416 and the surge tank 418 are connected in series and communicate with one another.

The pulse generator 402 can be, for example, a loudspeaker-like structure, or a piston, as is known in the art. The pulse generator 402 causes an appropriate fluid that is compressible (e.g., hydrogen gas, helium gas, neon gas, nitrogen gas) to oscillate in the cryocooler 400. The pulsetube cryocooler 400 uses the compression and expansion of the fluid to produce refrigeration by transferring heat from the cold heat exchanger 412 to the ambient heat exchanger 410.

In some applications where pulsetube cryocoolers are employed, space is at a premium. For example, pulsetube cryocoolers may provide cooling for electronics and the like on board satellites or extraterrestrial spacecraft or in a sensor pod on an aircraft. In particular, a pulsetube cryocooler can be used to cool infrared sensors on satellites, where space is scarce and weight is an important consideration. In such applications, the space occupied by a surge tank and the weight of the surge tank must be minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying figures where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.

FIG. 1 is a schematic block diagram of a satellite that includes a pulsetube cryocooler, which includes a surge tank;

FIG. 2 is a diagrammatic cross sectional view of the surge tank of the cryocooler of FIG. 1;

FIG. 3 is a graph showing specific power as a function of tank volume and a comparison between an open tank and a packed tank; and

FIG. 4 is a schematic block diagram of a prior art pulsetube cryocooler.

SUMMARY

Basically, the invention is a cryocooler including a surge tank, wherein the tank encloses a space, and fibrous packing material is distributed substantially throughout the space enclosed by the tank.

In another aspect of the invention, the cryocooler is a pulsetube cryocooler and further includes: a regenerator; a pulsetube, which is coupled to the regenerator; and an inertance tube, which is located between the pulsetube and the surge tank.

In another aspect of the invention, the packing material is glass or metal wool.

In another aspect of the invention, approximately 5 to 10 percent of the volume of the space enclosed by the tank is occupied by the packing material.

In another aspect of the invention, the packing material is glass or metal wool.

In another aspect of the invention, approximately 5 to 10 percent of the volume of the space enclosed by the tank is occupied by the packing material.

In another aspect of the invention, the cryocooler is part of an aircraft, a satellite or a spacecraft.

In another aspect, the invention is a surge tank of a cryocooler, and fibrous packing material is distributed substantially throughout a space enclosed by the surge tank to make compression and expansion of gas within the surge tank relatively more isothermal as compared to the surge tank when empty.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an aircraft, a satellite or spacecraft 100 on which a pulsetube cryocooler 101 is mounted. The cryocooler 101 includes a pressure wave generator 102, or compressor, a regenerator 104, an ambient heat exchanger 110, and a cold heat exchanger 112. The pressure wave generator 102 is coupled to one end of the regenerator 104. Coupled to another end of the regenerator 104 is a pulsetube 114. An inertance tube 116 is coupled to the pulsetube 114 as shown. A surge tank 118, or gas spring, is coupled to the inertance tube 116 such that the inertance tube 116 joins the surge tank 118 to the pulsetube 114. As shown in FIG. 1, the pressure wave generator 102, the regenerator 104, the pulsetube 114, the inertance tube 116 and the surge tank 118 are connected in series and communicate with one another.

The pressure wave generator 102 can be, for example, a piston coupled to voice coils. The pressure wave generator 102 causes an appropriate compressible fluid (usually helium gas) to oscillate in the cryocooler 101. The pulsetube cryocooler 101 uses the compression and expansion of the fluid to transfer heat from the cold heat exchanger 112 to the ambient heat exchanger 110 in a known manner to refrigerate a space.

The surge tank 118 serves as a gas spring. That is, the surge tank 118 provides compliance. A basic equation for a gas spring indicates that the compliance is proportional to V/γ where V represents volume and γ=1.66 for adiabatic compression of helium. For isothermal compression of helium, γ=1.0. In the conventional surge tank 418 (FIG. 4), which is empty, the compression is adiabatic. Thus, in the conventional surge tank 418 (FIG. 4) where helium is the fluid, γ=1.66.

As shown in FIG. 2, the surge tank 118 of the illustrated embodiment includes fibrous packing 210. The packing 210 is distributed throughout substantially the entire interior space of the surge tank 118. The packing 210 causes the gas within the surge tank 118 to compress more isothermally (compared to an empty tank). Thus, in the surge tank 118 of the preferred and illustrated embodiment, γ=1.0. If the porosity of the packing 210 in the surge tank 118 is 90%, then the effective volume change is represented by the following equation:

0.9×(1.66/1.0)≈1.5

Thus, in theory, the use of the packing 210 permits the volume of the reservoir 118 of the illustrated embodiment to be smaller than the surge tank 418 (FIG. 4) of the conventional cryocooler by a factor of approximately 1.5 while yielding substantially the same spring performance.

The packing 210 can be, for example, glass or metal wool 212. The preferred diameter of the wool ranges from approximately 1 micron to approximately 20 microns. Such wools are commercially available from several sources including a German company known as Bekaert. The wool 212 makes the compression of the fluid more isothermal, which improves the performance of the surge tank 118.

In particular, the wool 212 creates more heat transfer area. When the gas in the reservoir 118 is compressed, it heats, and heat is transferred to the wool 212. The transfer of heat to the wool 212 shrinks the gas, which is desirable in the compression stage for good gas spring performance. On the other hand, when the gas in the reservoir 118 expands in the course of the gas oscillation, the gas cools and heat from the wool 212 is transferred to the gas. This expands the gas, which is desirable at the expansion stage of the gas for good spring performance. Thus, the wool 212 improves the springiness of the gas and allows the surge tank 118 to perform like a larger empty reservoir such as the reservoir 418 of FIG. 4.

The wool 212 is distributed throughout the entire volume, or interior space, of the surge tank 118, but it is very porous so does not occupy a significant amount of the tank volume. In order to get sufficient heat transfer and thermal storage, about 5% to 10% by volume of wool material should be used. Thus, the wool is 90% to 95% porous. The wool should have small fiber diameter (˜1 μm to 20 μm) for good heat transfer characteristics. Metal fiber wools (˜6 μm fiber diameter) in 90% to 95% porosity, are commercially available.

FIG. 3 shows the result of a computer simulation in which two surge tanks are compared. A first tank, which is represented by square points on the left-hand graph of FIG. 2, was packed with packing material such as wool 212. The second tank was empty and is represented by diamond-shaped points on the right-hand graph of FIG. 2. The porosity of the empty tank was considered to be 99.9%. The porosity of the packed tank was 90%. The graph of FIG. 3 shows that, in a cryocooler, a surge tank having a volume of approximately 12 cc and that is packed with fibrous packing material has a spring performance that is approximately the same as an empty surge tank that has a volume of approximately 20 cc.

The graph of FIG. 2 illustrates plots of specific power as a function of tank volume. The specific power values represent the number of watts of power input to the cryocooler divided by the number of watts of refrigeration produced. Thus, the minimum points of the graph are optimal. As shown by the graph, the specific power is approximately the same (about 25.35) for both the empty tank and the packed tank. However, since the volume of the packed tank (the first tank) is only 12 cc, the simulation shows that the packing permits the size of the surge tank to be reduced while maintaining the same performance. By varying the fiber diameter of the packing and by varying the material making up the packing, further reductions in volume are possible. Thus, the packing 210 allows a volume and weight reduction of the surge tank 118 without sacrificing performance.

Although this invention has been described in the context of a cryocooler, the surge tank can be used in other applications where a gas spring is required and size or weight is a concern.

It is believed that the foregoing description of a preferred embodiment of the invention is sufficient in detail to enable one skilled in the art to make and use the invention. However, it is expressly understood that the detail of the elements presented for the foregoing purpose is not intended to limit the scope of the invention, in as much as equivalents to those elements and other modifications thereof, all of which come within the scope of the invention, will become apparent to those skilled in the art upon reading this specification. Thus, the invention is to be broadly construed within the full scope of the appended claims. 

1. A cryocooler comprising a surge tank, wherein the tank encloses a space, and fibrous packing material is distributed within the space enclosed by the tank.
 2. The cryocooler of claim 1 wherein the packing material is glass or metal wool.
 3. The cryocooler of claim 1 wherein approximately 5 to 10 percent of the volume of the space enclosed by the tank is occupied by the packing material.
 4. The cryocooler of claim 1 further comprising: a regenerator; a pulsetube, which is coupled to the regenerator; and an inertance tube, which is located between the pulsetube and the surge tank.
 5. The cryocooler of claim 4 wherein the packing material is glass wool.
 6. The cryocooler of claim 4 wherein the packing material is metal wool.
 7. The cryocooler of claim 4 wherein approximately 5 to 10 percent of the volume of the space enclosed by the tank is occupied by the packing material.
 8. The cryocooler of claim 1, wherein the cryocooler is part of an aircraft, a satellite or a spacecraft.
 9. The cryocooler of claim 1 wherein the packing material is distributed entirely throughout the space enclosed by the tank.
 10. A cryocooler comprising a surge tank, wherein fibrous packing material is distributed substantially throughout a space enclosed by the surge tank to make compression and expansion of gas within the surge tank relatively more isothermal as compared to the surge tank when empty.
 11. The cryocooler of claim 10 wherein the packing material is glass wool.
 12. The cryocooler of claim 10 wherein the packing material is metal wool.
 13. The cryocooler of claim 10 wherein approximately 5 to 10 percent of the volume of the space enclosed by the tank is occupied by the packing material.
 14. The cryocooler of claim 10, wherein the cryocooler is part of an aircraft, a satellite or a spacecraft.
 15. The cryocooler of claim 10 wherein the packing material is distributed entirely throughout the space enclosed by the tank. 